(i) Field of the Invention
The present invention relates to an optical communication system using an optical fiber.
(ii) Description of the Related Art
In a conventional optical transmission system, for example, in the case of a single wavelength transmission, a signal of one optical channel using one wavelength is optically modulated and transmitted to one optical fiber. In this case, when the optical fiber is used for a service, whether the optical channel is available can be confirmed by detecting a deterioration in optical signal which is actually transmitted or an input disconnection of the signal. When the optical fiber is not used for the service (it is not under operation), the optical signal of the wavelength which is used for the service is experimentally transmitted and the connection is confirmed, so that the availability of the optical channel can be confirmed.
In the case of a wavelength division multiplexing (WDM) transmission in which different wavelengths are allocated to a plurality of optical channels and those are multiplexed to be transmitted, when the optical channels are used for the service, the availability of each of the optical channels (wavelengths) can be recognized by detecting the deterioration in optical signal of the wavelength in question which is actually transmitted or input disconnection of such a signal. For the wavelength not under operation, which is not used for the service, whether it is available or unavailable is estimated by detecting the availability of the wavelength under operation for the service. In the case of before the start of the service operation, for instance, just after laying the optical fiber, one proxy wavelength among a plurality of wavelengths is selected and the connection of the wavelength is confirmed, so that it is determined that all of the wavelengths are available.
As mentioned above, in the case of the single-wavelength transmission, when the line not under operation is switched so as to be used for the service, to assure a communication quality, the operator of a communication system (generally, the maintenance person for communication carriers) must previously perform a connection test to confirm the availability and, after that, switch the corresponding optical fiber to be in the service operation state. It is cumbersome for the maintenance person.
Also in the case of the WDM transmission, on the basis of a specific wavelength under operation for the service, the availability of the optical channel not under operation is estimated. It does not mean that the actual availability of each of the wavelengths which can be used for the service is confirmed.
On the other hand, in ITU-T (International Telecommunication Union), the available quality of the transmission is defined as shown in FIG. 3. FIG. 3 illustrates the available quality matrix according to ITU Recommendation I357. When it is applied to the wavelength division multiplexing transmission, in the case where the wavelength of a specific optical channel is in the non-operating state (hereinbelow, referred to as an optical channel not under operation), there is no means of determining whether the optical channel not under operation is available, namely, it denotes a state (3) in FIG. 3 during the system operation. Therefore, when it is desired to operate the optical channel not under operation because of any reason, in order to determine whether the channel is surely in the available state and then use it, it is necessary that the system operation is temporarily stopped, the optical channel is checked, after that, the optical channels are switched, and a procedure for the start of the operation is again performed.
The present invention solves the above-described problems. According to the invention, there are provided a method and an apparatus whereby even when a specific optical channel is not under operation, the availability of the optical channel can be voluntarily measured.
To solve the above problems, an optical transmission system of the present invention has on a transmitter side a connection confirming circuit for confirming connection of an input data signal, a test signal generator for generating a test signal, and a switching device. The switching device has an input terminal to which the input data signal is supplied, an input terminal to which the test signal is supplied, and an output terminal and selectively generates the input data signal from the output terminal when the input data signal is detected by the connection confirming circuit, and the test signal when the input data signal is not detected. An optical transmitter for converting the signal outputted from the switching device into an optical signal of a predetermined wavelength and generating it to a transmission path is provided for the output terminal of the switching device. On a receiver side, there are provided a test signal detection circuit for receiving the optical signal from the transmission path to detect the test signal and a determination circuit for determining transmission characteristics of the transmission path on the basis of the test signal detected by the test signal detection circuit.